1. Field of the Invention
This invention relates to thermostats for controlling heating or air conditioning units, and more particularly relates to electronic wall thermostats for controlling the temperature in a room.
2. Description of the prior art
The use of electronic wall thermostats is well established and can be classified as either the low voltage switching type, typically 24 volts AC, for operating hydronic, steam or hot air systems, or line voltage switching types, for switching power to electric or other types of heaters.
Prior to the use of digital electronics in wall thermostats, conventional wall thermostats were typically of the bimetallic type utilizing the differential expansion of dissimilar bonded metals to actuate a switch, such as by tilting a mercury contact bulb switch. The inherent mass of the bimetallics prevents the conventional thermostat from quickly responding to temperature changes. As a result, large temperature swings occur.
Digital electronic thermostats that employ low mass sensors are able to respond more quickly to temperature changes and cycle the heating on and off to effect a lower room temperature differential and improve the comfort level. Without the relatively large temperature swings associated with bimetallic thermostats, energy savings using the digital electronic thermostats are greatly increased.
Significant energy savings may be realized by using day/night settings, reducing the room temperature at night or during periods of low usage. In some devices, a combination clock-thermostat is used to turn the heating setting back for expected low usage times, such as at night. To the knowledge of the inventor herein, most clock-thermostats use batteries to operate the clock of the thermostat. The batteries also provide power for the programming the thermostat and memory storage.
Some problems arise from low battery conditions beyond the obvious need to remember to periodically change the batteries. A low voltage condition can affect the calibration of the thermostat and eventually cause a false or ambiguous operation. This condition is discussed in U.S. Pat. No. 5,196,781, which issued to Robert Jamieson and John Weiss, the disclosure of which is incorporated herein by reference.
To avoid temperature control problems associated with battery aging or weak batteries, most low voltage thermostats still rely on bimetals for temperature controls and use the digital electronics for the clock and programming functions. Thermostats that depend on line voltage or a lower voltage as the power source may lose the temperature set and setback program during a brief power interruption, even if this interruption is only one or two seconds in duration. If line voltage thermostats use power storage devices, such as capacitors or battery back ups, then faulty operation or lock up may occur if the power is restored from a low or ambiguous voltage state.